The present invention relates generally to optical devices and more particularly to micromachined tunable optical filters and techniques for fabricating such filters.
Tunable optical filters are useful devices for wavelength-division-multiplexing (WDM) systems, performing functions such as optical monitoring, channel selection in wavelength-based routing, noise filtering and coherent crosstalk reduction. As the number of wavelengths used in these systems grows, it is particularly desirable to have inexpensive tunable filters. Existing tunable filters have a relatively high unit cost, due to the labor-intensive fabrication and assembly processes which are used. Among these, tunable Fabry-Perot (FP) filters based on mechanical scanning of a FP cavity length are generally best suited to meet the high performance required in WDM systems, due to important optical properties such as, for example, low loss, polarization insensitivity, large tuning range and high bandwidth resolution. In addition to the problem of high cost, the tuning speed of bulk mechanical filters is typically rather slow, i.e., on the order of milliseconds, as the tuning process requires moving a relatively large mass.
A tunable FP filter is characterized by a cavity enclosed between two mirrors. The transmission function of a symmetric FP filter with identical mirrors is given by:                     T        =                                            (                              1                -                R                            )                        2                                                              (                                  1                  -                  R                                )                            2                        +                          4              ⁢                              xe2x80x83                            ⁢              R              ⁢                              xe2x80x83                            ⁢                                                sin                  2                                ⁡                                  (                                      δ                    /                    2                                    )                                                                                        (        1        )            
where R is the mirrors"" power reflectivity and xcex4 is the accumulated phase a light wave acquires in each round-trip inside the cavity, given by:                     δ        =                              4            ⁢            π            ⁢                          xe2x80x83                        ⁢            nL                    λ                                    (        2        )            
Here, n is the index of refraction of the material comprising the cavity (n=1 for air), L is the cavity length, and xcex is the operating wavelength. The resonant wavelengths of this filter are determined by the phase xcex4 given above, and the separation between the wavelengths, called the free-spectral-range (FSR), is given approximately by:                               Δ          ⁢                      xe2x80x83                    ⁢                      λ            FSR                          =                              λ            2                                2            ⁢            nL                                              (        3        )            
The passband width of the resonant peak is determined by the filter finesse F, which is a measure of the overall cavity and mirror losses:                     δλ        =                              Δλ            FSR                    F                                    (        4        )            
For an ideal lossless filter, the finesse is given by F=xcfx80R/(1xe2x88x92R). The wavelengths that the filter transmits can be tuned, among other ways, by mechanically tuning the cavity length.
One type of conventional optimized tunable filter design approach sets the free spectral range to be about equal to the required tuning range. For WDM systems, a typical tuning range is in the range of 40-100 nm, and the center wavelength is approximately 1.55 xcexcm. Using Eq. (3), this translates to a cavity length of 10-30 xcexcm. If the WDM system uses 0.8 nm (100 GHz) channel spacing, a tunable filter used as a channel selector would require a filter bandwidthxe2x89xa60.5 nm, and from Eq. (4), a finesse of 80-200. This means that the required mirror reflectivity would be close to 98-99%.
In order to reduce cost substantially and also to enable faster switching speeds, micromachined FP filters have been developed. Examples of micromachined FP filters are described in M. C. Larson, and J. S. Harris Jr., xe2x80x9cBroadly-tunable resonant-cavity light-emitting diode,xe2x80x9d IEEE Photon. Technol. Lett., Vol. 7, p. 1267, 1995; E. C. Vail et al., xe2x80x9cGaAs micromachined widely tunable Fabry-Perot filters,xe2x80x9d Electron. Lett., Vol. 31, p. 228-229, 1995; P. Tayebati et al., xe2x80x9cWidely tunable Fabry-Perot filter using Ga(Al)Asxe2x80x94AlOx deformable mirrors,xe2x80x9d IEEE Photonic Technol. Lett., Vol. 10, pp. 394-396, 1998; J. Peerlings et al., xe2x80x9cLong resonator micromachined tunable GaAsxe2x80x94AlAs Fabry-Perot filter,xe2x80x9d IEEE Photonic Technol. Lett., Vol. 9, pp. 1235-1237, 1997; and A. Spisser et al., xe2x80x9cHighly selective and widely tunable 1.55 xcexcm InP/air-gap micromachined Fabry-Perot filter for optical communications,xe2x80x9d IEEE Photonic Technol. Lett., Vol. 10, pp. 1259-1261, 1998.
These micromachined filters share a common design approach, which defines vertically the entire FP structure, including its cavity and mirrors, by a sequence of multi-layer thin-film depositions on a wafer substrate. In this design approach, both top and bottom cavity mirrors are typically comprised of several quarter-wave-thick layers with alternating high and low refractive indices, while the layer which is used to define the cavity is a sacrificial layer which is later etched away in one of the final processing steps. The etching process forms a membrane or a cantilever structure. Cavity tuning is obtained electrically by pulling the membrane or the cantilever toward the substrate with electrostatic force, which changes the cavity spacing between the mirrors.
The vertical design approach for micromachined filters restricts the initial cavity length to the thickness of the sacrificial layer, which is limited in most cases to only a few microns at the most. This results in a very large spacing between the periodic transmission peaks of the filter, and as a consequence, a very high mirror reflectivity is required to obtain a filter bandwidth which is narrow enough to meet dense WDM requirements. A typical cavity length in this type of micromachined filter may vary in the range of 2-5 xcexcm, which translates to a FSR of 250-600 nm. To achieve a filter bandwidth of 0.5 nm, the required finesse and mirror reflectivity are 500-1200 and 99.5-99.8%, respectively.
Such high mirror reflectivities can be obtained with a large number of quarter wave mirrors, or alternatively by substantially increasing the index contrast between the layers, as described in, e.g., U.S. Pat. No. 5,739,945 issued to P. Tayebati and entitled xe2x80x9cElectrically tunable optical filter utilizing a deformable multi-layer mirror,xe2x80x9d and the above-cited A. Spisser et al. reference. Either of these approaches, however, unduly complicates the fabrication process. Furthermore, the filter is exposed to substantially higher throughput losses in the presence of any type of defect or deviation from an ideal FP structure, e.g., mirror curvature or tilt, and intracavity diffraction of a non-collimated illumination. As the channel spacing of WDM systems becomes even smaller, the requirements on the filter finesse and reflectivity become even more difficult to achieve with this short cavity design. It should also be noted that since the absolute change in cavity length required to shift the filter passband over a certain wavelength range is a linear function of the initial cavity length, shorter cavities are more sensitive to small fluctuations in cavity length. For typical micromachined filters with cavity length of only a few microns, a cavity length change on the order of 0.1 nm (1 xc3x85) would shift the filter passband by 0.1 nm, making it difficult to stabilize the filter transmission wavelength.
As noted previously, a conventional non-micromachined design approach involves choosing the transmission peak spacing to be about equal to the required tuning range, such that a typical WDM requirement in the range of 40-100 nm translates to cavity spacing of 10-30 xcexcm, and respective mirror reflectivities of 98-99%. In an attempt to achieve this type of cavity spacing for a micromachined tunable filter, a design disclosed in the above-cited J. Peerlings et al. reference patterns the top and bottom mirrors separately on two different substrates, and then assembles them together with a cavity spacing that is defined by fixed spacers between the substrates. In the disclosed device, the cavity length was 30 xcexcm and the resulting filter periodicity was 56 nm. The obtained filter bandwidth was higher than expected from the mirror reflectivity, only 1.2 nm instead of about 0.5 nm, due to mirror tilt which probably developed during the assembly. However, this type of design also has a number of drawbacks. The additional assembly step adds to the device cost, since careful assembly is required to avoid degradation in device performance. In addition, two substrates need to be processed rather than one, and since tuning speed is determined by the thickness of the structure that is being pulled electrostatically, one of the substrates requires substantial thinning to allow for reasonable tuning speeds on the order of milliseconds.
It is apparent from the above that a need exists for an improved micromachined tunable optical filter which can provide a cavity length which is not unduly limited, e.g., to the thickness of the sacrificial layer, without the problems associated with using multiple substrates to form a single filter.
The present invention provides tunable micromachined optical filters in which cavity length is not subject to the undue restrictions generally associated with the above-described prior art devices. A tunable optical filter in accordance with the invention incorporates a first surface-micromachined out-of-plane plate having a moveable membrane which includes a first high reflective (HR) coated mirror. The mirror defines one side of a Fabry-Perot (FP) filter cavity and is movable in a direction along an axis of the filter cavity. The other side of the filter cavity is defined by a second HR-coated mirror. The filter cavity may be defined horizontally or vertically, depending on the configuration and arrangement of the second mirror relative to the first mirror.
In a first illustrative embodiment of the invention, the first plate is formed on a substrate, and subsequently released from the substrate and secured in a plane orthogonal to the substrate. The first HR-coated mirror is formed as part of the movable membrane supported in an opening through the first plate. The second mirror is part of a second plate which is formed on the substrate, and subsequently released from the substrate and secured in another plane orthogonal to the plane of the substrate, such that the filter cavity is defined horizontally between the first and second mirrors. As voltage is applied between an electrode associated with the moveable membrane and a facing electrode on the second plate, the first mirror is pulled toward the second plate, thus changing the cavity length in accordance with the applied bias level. This in turn tunes the transmission peaks of the filter to varying wavelength positions.
In a second illustrative embodiment, the second mirror is formed on an endface of an external fiber, such that light from the fiber can pass from the second mirror through an opening in the second plate to the first mirror. The filter cavity in this embodiment is also defined horizontally, but between the first mirror of the first plate and the second mirror on the fiber endface. The bias that controls the membrane displacement is applied between the membrane electrode and a second electrode which can be positioned anywhere between the two mirrors, or even outside the cavity, as long as it does not block the light path inside the cavity. For example, the second electrode can be defined on the second plate, or on any other structure, including structures that are not micromachined out of the substrate but are instead attached manually to the substrate or other suitable surface.
In a third illustrative embodiment, the second plate is eliminated and the second mirror is formed on the substrate. The first plate is then arranged over the second mirror, in a plane parallel to the plane of the substrate, and separated from the substrate by spacers, such that the filter cavity is defined vertically. The cavity length is determined initially by the spacers, which can be defined using various techniques, e.g., with surface micromachining or by external application to the substrate. One possible technique is to use solder bumps both as spacers and as a mechanism to provide electrical connection between the first plate electrodes and corresponding electrodes formed on the substrate. The moveable membrane associated with the first plate is used as a tuning mechanism, as in the other illustrative embodiments.